1. Field of the Invention
The present invention relates to a method for densifying a transmitter and receiver network for mobile telephony.
Although the present invention can be applied to different types of mobile telephone systems, it will be described in the following with respect to a GSM mobile telephone system.
2. Description of the Related Art
The capacity of a mobile telephone system is restricted by interferences between different transmitter and receiver units. Each transmitter and receiver unit includes an antenna for transmitting and receiving signals to and from a mobile telephone. Communication is effected on different predetermined frequencies, i.e. channels.
Two different types of interference must be taken into account when planning an antenna network, particularly in municipal environments. Interference between two channels on one and the same frequency is one type of interference. In the case of the GSM system, this type of interference means that the carrier signal must be stronger than the interference signal by more than 9 dB. When this criterion is not fulfilled, speech quality is poor and there is a risk of the call being lost, i.e. broken-off.
The other type of interference is caused by the carrier signal being disturbed by a closely adjacent channel. The different permitted channels are numbered in the GSM system. For instance, channel 72 may be disturbed by either channel 71 or 73. In the case of this type of interference, the carrier signal must not be weaker than a closely adjacent channel by more than 9 dB in the GSM system.
When planning a network of antennas operating on different channels, it is necessary to take the aforesaid interferences into account. This applies primarily to municipal environments, in which the antennas are placed relatively close together so as to make achievable a high traffic intensity with regard to the number of simultaneous calls.
There are a number of different accepted models on which a network can be based. The majority of these models include so-called three-sector sites, meaning that a base station is equipped with three directional antennas, normally 60-degree antennas, the directions of which are mutually spaced by 120 degrees. Each antenna is supplied with one or more channels and forms a so-called geographic cell. The antennas belonging to a base station are supplied with different frequencies. The base stations are positioned in accordance with a pattern, in which the cells form an hexagonal configuration. Which channels are transmitted in which antennas is determined by the frequency pattern chosen.
A typical frequency pattern is a 4/12-pattern. In this case, all available frequencies are used once on four base stations including twelve cells. Positioning of the base stations and the cell frequencies are repeated in a repetitive pattern with the cells forming said hexagonal configuration, so that each cell that has one particular frequency will be spaced as far as possible from an adjacent cell that has the same frequency. In other words, this requires at least twelve channels are needed to obtain a 4/12-pattern with one channel per cell. Each cell can be supplied with two channels, provided that twenty-four channels are available.
When capacity is deficient, a denser frequency pattern can be chosen. One such frequency pattern is a 3/9-pattern, meaning that all available frequencies have been used once on three base stations which include nine cells. Thus, in order to increase capacity in comparison with a 4/12-pattern with twenty-four channels, twenty-seven channels are used with each cell being equipped with three channels. There is a great deal of uncertainty with regard to the function of a 3/9-pattern, because of the serious risk that interference problems will occur with subsequent poor speech quality.
A further pattern, namely a 2/12-pattern, has been described. In this pattern, each base station has six sectors and the channels end up in accordance with a given pattern. The 2/12-pattern is highly prone to interference problems.
When needing to increase the capacity of a mobile network, a number of different measures can be taken. The first step in this regard is to add a channel, provided that further channels are available. In the case of a 4/12-pattern, the access to thirty-six channels would mean that three channels can be used with each cell.
Provided that twenty-four channels are used in a 4/12-pattern, capacity can be increased by switching to a 3/9-pattern without needing to build new base stations. A 3/9-pattern, which requires twenty-seven channels, cannot be used to the full when only twenty-four channels are available. However, this means that the frequencies are repeated more frequently, which leads to quality impairment.
Another method employed in this regard involves the use of microcells. Microcells are small base stations of limited range, placed on the walls of buildings for instance, about 5-10 meters above street level. A large number of microcells is able to relieve a superordinate network, i.e. the standard base stations. One drawback with microcells is that they cover only a small area, which results in a variation in signal strength on the part of a user who moves quickly between the cells, since large variations in signal strength occur when turning round street corners. Another drawback resides in the cost of each microcell. The indoor coverage afforded by a microcell is also poor.
The present invention solves the problem of markedly increasing the capacity of the network within a desired geographical area, with a limited number of available channels.